This invention relates to an apparatus and method for providing an aero-optical interface by controlling a shear layer in a fluid flowing past a cavity having an aperture opening into a surface. It has particular but not exclusive application in the area of providing uniform, low-loss optical transmission to and from an aircraft through a shear layer such as in radiation and laser window design.
In the relatively new field of aero-optics, the goal is to obtain a high-quality optical window which affects radiation propagating therethrough as little as possible. Typically, apertures opening into cavities are placed in the surfaces of aircraft for transmission and receipt of optical wavelength radiation. The air flowing past such an aperture creates a shear layer starting at the upstream edge or so-called separation corner of the aperture. Transmission through this layer is governed by spatial and temporal variation in the index of refraction, the scale sizes of the variations, the layer thickness, and the optical wavelength. This shear layer, which thickens along the streamwise length of the aperture, reduces the pressure within the cavity through a viscous turbulent flow entrainment pumping action. The reduced pressure within the cavity deflects part of the shear flow into the cavity. An equilibrium condition is attained when the shear flow mass deflected into the cavity just equals the mass lost through viscous pumping. The process can be conceived of as part of the shear layer being deflected and peeled off into the cavity, recirculating through the cavity, and rejoining or "feeding" the underside of the shear layer. Because this process is, by itself, unstable, there are strong fluctuations in the position of the shear layer and in the pressure levels inside the cavity.
In addition to causing an undesirable cavity environment, the unsteady flow causes losses in the uniformity and intensity of transmitted optical radiation. These losses are the same whether the radiation is directed outward from the aircraft, as from an aircraft-mounted laser, or inward into the aircraft, as in an aircraft-mounted sensor. Non-uniformities in transmitited intensity and net angle of refraction in such arrangements are caused by scattering of radiation as it passes through the shear layer due to local variations in the shear layer refractive index, which are in turn a product of both small and large scale variations in pressure, turbulence, and velocity distribution. Transmitted optical quality losses are exacerbated by the thickening of the shear layer and increases in turbulence scale size as it trails away from the leading (upstream) edge of the aperture unless there is a corresponding decrease in the intensity of refractive index variations.
One known technique for attempting to create an optical-quality shear layer involves placing a partially porous barrier or "fence" just upstream of the aperture. Such a technique is used in the NASA Ames Airborne Observatory, and described in D. A. Buell's paper entitled, "Airloads Near the Open Port of a One-Meter Airborne Telescope," presented at the AAIA 13th Aerospace Sciences Meeting, (1975). The flow deflection by the fence displaces the shear layer relative to the downstream end of the opening and reduces pressure over the cavity. The fence, in effect, isolates the shear layer from the cavity by providing air to the underside of the shear layer from the freestream through the porous fence rather than from the cavity. Fences are typically up to one-quarter the aperture length in height and are about 50% porous. While the porous fence technique has proven excellent for controlling the pressure across fluid mechanical exit ducts, it does so by thickening the shear layer, adding relatively large scale turbulence, and inhibiting the development of shear flow self-similarity. These three effects make the porous fence technique unsuitable as a technique for obtaining an aero-optical interface due to resultant increased scattering losses in intensity and non-uniform refraction. This is especially true at freestream Mach numbers above about 0.5 due to the formation of locally supersonic flow which extends the shear layer out to the top of the recompression shock which is unstably extended above the fence.
In short, the prior art methods, such as the fence or cavity alone are limited in usefulness to very low density and/or long wavelength optical transmission because, in general, they provide thickened, increased turbulence as well as non-self-similar and unsteady shear flow distributions, and unsteady shear flow reattachment.